1. Field of the Invention
This invention relates to a hydraulic system for controlling the cutting feed speed of a saw frame of a sawing machine, such as a band-sawing machine. More particularly, the present invention relates to a hydraulic system for controlling the cutting feed speed of a saw frame in response to changes of cutting resistance to the saw blade occurring in accordance with varied widths of the cut, thereby protecting the saw blade against overloading by the saw frame.
2. Description of the Prior Art
The common practice is to provide a sawing machine with a hydraulic system for controlling the cutting feed speed of a saw frame. Normally two different methods are adopted depending upon the situations; one is that the saw frame is slidably carried on a pair of columns erected on the base, and the other is that it is pivotally supported on the base at its one end while the other end portion is free to move up and down. These two methods are fundamentally identical, in that each has a hydraulic cylinder in which a ram is reciprocated under hydraulic pressure, the ram including a rod fastened to the saw frame. In this system, the saw frame is raised when the oil is pumped into the cylinder from the reservoir, and the same lowers rapidly when the oil is returned to the reservoir. In this case, the conventional system employs a directional valve in the return path, thereby resulting in a direct hydraulic return to the reservoir. But, when the oil is returned to the reservoir through the directional valve and a throttle valve, the lowering speed of the saw frame is very gradual, which is commonly called a cutting feed. This cutting feed is constant irrespective of variations in the cutting width of the work. However, the constant cutting feed speed endangers the saw blade to overloading, and lessens the efficiency.
Therefore, there has arisen a demand in the industry for ensuring an adequate cutting feed speed thereof under hydraulic systems. Many proposals have been made for this purpose, among which is that a control valve is provided in the return path of oil, between a directional valve and the discharge, whereby the oil pressure is reduced from the larger first pressure to the smaller second pressure. In addition, a throttle valve is provided between the control valve and the discharge, so as to ensure that the oil flow can pass the throttle valve at the rate corresponding to the secondary pressure of the control valve. As commonly known in the art, the pressure exerting on the inlet port of the throttle valve is in a proportional relationship with the rate at which the oil passes through the throttle valve. Under this structure, the sum of the pressure in the cylinder and the cutting resistance to the saw blade can be constantly equal to the weight of the saw frame, wherein the "cutting resistance to the saw blade" means a vertical load upon the saw blade. However, the cutting resistance to the saw blade is not always constant. For example, when the work to be cut is an H-beam, the cutting amount changes during the operation, which leads to changes of the cutting resistance to the saw blade. In order to avoid a problem due to changes of cutting resistance, it has been proposed that by detecting changes of cutting resistance to a saw blade as those of the pressure in the cylinder, the discharge of oil in the cylinder is controlled. Thus, when the pressure in the cylinder is high, the exhaust of oil therefrom is increased, thereby speeding up the cutting feed speed of the saw frame. In contrast, when it is low, the exhaust of oil therefrom is reduced, thereby slowing the cutting feed speed of the saw frame. This system is particularly applicable for small-size cutting machines, but when the machines are large-sized, difficulties have been brought about, due to the fact that the saw frame is so heavy for the cutting resistance to the saw blade that responses therefrom are likely to be negated, thereby losing control over the cutting feed speed. This causes damages to the saw blade. For precaution, therefore, it can be proposed that the cutting feed is maintained at a slow speed throughout the operation, but it is inefficient.
In order to avoid such troubles, Japanese Patent Specification Publication No. 52-10748 (published on Mar. 25, 1977) proposes one solution by providing a device for detecting any distortions occurring on the saw blade on each arm supporting the same. However, this method has a drawback that a practicably sensitive signals cannot be expected unless the distance between the pair of arms is considerably long. In addition, the required hydraulic system becomes complicated, thereby resulting in a low responsiveness, and what is more, the coolant supply line and the detecter line are likely to become entangled or twisted, thereby causing troubles during the cutting operation.
U.S. Pat. No. 4,016,787, whose invention was made by the inventor of the present invention, discloses a further expedient for detecting changes of cutting resistance to saw blade as changes of the pressure in the cylinder by means of a piezo-electric element, so as to control the discharge of oil in the cylinder, and therefore to maintain the cutting resistance to the saw blade at an optimum. Nevertheless, it has been found that this method also has drawbacks of complication and expensiveness because of the fact that the system must include an electric system as a controller.
The present invention is directed toward solving these problems pointed out above, and has for its principal object to provide an improved hydraulic system for controlling the cutting feed speed of a saw frame, without the use of any electric means but in the hydraulic system itself.